Turbochargers are a type of forced induction system. They deliver compressed air to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can allow for the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle. Turbochargers use the exhaust flow from the engine to drive a turbine, which in turn, drives the air compressor. To supply air outside of the capabilities of a simple system, the turbocharger system may take on various configurations.
Turbochargers consist of five major component groups. A section of a typical turbocharger is shown in FIG. 1. This turbocharger consists of a turbine housing (1) which is connected to a bearing housing (5). On the opposite end of the bearing housing is a compressor cover (2). The bearing housing supports a rotor assembly which consists of a turbine wheel (3) and a compressor wheel (4). The turbine housing is usually cast in a material from the ductile cast iron family, the bearing housing is usually cast in gray iron and the compressor cover is usually cast in an alloy of aluminum. The mass of a typical commercial Diesel size turbine housing is approximately 17 kg. These turbine housings have typical wall thicknesses of 5 to 6 mm. This is approximately 65% of the total mass of the turbocharger. The bearing housing is another 4 kg which makes the mass of the turbine housing, plus bearing housing, 85% of the total mass.
To achieve more boost, series or multiple stage turbochargers, where a first stage (low pressure) compressor discharges into the inlet of the next (high pressure) downstream compressor, which then further boosts the already compressed air to an even higher level, are used. Series turbochargers can have multiple stages but for the sake of clarity this discussion will discuss only two stage configurations, as that is the highest number used in common production in passenger car and commercial Diesel engines. The ratio “turbine housing plus bearing housing mass” to “total turbocharger weight” for multiple turbochargers will be a ratio similar of that for single turbochargers, so for example where the mass of turbine housing plus bearing housing of two turbochargers will be approximately 34 kg, this will still be 85% of the total mass of the turbochargers.
The thermal inertia of a body is a bulk material property related to the thermal conductivity, density and volumetric heat capacity of a material. Thermal inertia is used to describe the ability of a geometric body to come to thermal equilibrium when subjected to a change in its thermal environment. The resultant, in the context of an engine system, is the time it takes a geometrically defined body to change temperature. Masses being equal, a material with high thermal inertia will take more time to change temperature than a material of low thermal inertia. In the case of two objects of the same material but with different mass, the object with more mass will have more thermal inertia than the object with smaller mass (just as a cubic meter of cast iron will take longer to come to temperature equilibrium than would a cubic millimeter). In the context of the present invention, the term “thermal inertia” is used to describe the inherent dynamic temperature filtration, i. e., the relatively slow coming to equilibrium from an initial temperature differential existing between the exhaust gases and the material in the engine and the exhaust system. This thermal inertia is in turn due to the heat transfer between gas and wall material, the volumetric heat capacity of the material, which involves the thermal conductivity of the material, the specific heat and the density of the material and the thermal effect of the surrounding media (e.g. air, water and material).
The thermal inertia of a body is calculated by the formula:Thermal Inertia=κ·ρ·C    Where κ=the bulk thermal conductivity of the material in Wm−1·K-1    and ρ=the density of the material in Kg m−3     and C=the specific heat capacity in J·kg−1·K−1     The units are tiu.
In a calculation of thermal inertia, for a given material, for example cast iron, the specific heat and density will remain a constant. Therefore the thermal inertia is proportional to the bulk thermal conductivity of the material.
Since thermal conductivity is a measure of the quantity of heat transmitted over a period of time Δt, through a material of thickness Y and area A, across a temperature change of ΔT, with a heat energy change of ΔQ, the thermal conductivity κ can be calculated through the formula:
  κ  =            Δ      ⁢                          ⁢              Q        ·        Y                    Δ      ⁢                          ⁢              t        ·        A        ·        Δ            ⁢                          ⁢      T      
If in this discussion the amount of heat energy supplied and the temperature change ΔT are considered to be constants, the variables are then the area of the material “wetted” by the exhaust gas, the thickness “Y” in a direction perpendicular to the plane of the area “A” of the material, and the time “t” for the body to come to thermal equilibrium. In turbine housings and other similar castings the actual thickness of the thinnest sections is often driven by foundry practices. In turbine housings, the technical requirement for material thickness is less than the casting practice minimum which is in the regime of 6 mm to 8 mm. This means that over the myriad of features in a turbine housing, the surface area “A” is the predominant variable which determines a change in thermal inertia.
If one considers the thickness to be a constant average thickness of 10 mm, then the surface area becomes directly proportional to the volume of material. With the density of a given material being constant for that material, the surface area becomes directly proportional to the mass (since mass=volume×density).
Since typical turbochargers weigh in the range of 2 to 35 kg the volume of material in the casting will cause a lag in the time for the system to change temperature during a transient in engine conditions. The volume of material in the turbocharger assembly pulls thermal energy from the exhaust gas, which results in lower temperature exhaust gas downstream of the turbocharger assembly.
The properties of various turbocharger materials are:
kCThermalSpecificρconductHeatDensityw/(mK)J/KgKKg/m3Cast Iron (Pearlitic)33.4726037100SS 30915.665029010Aluminum A3561289002680Al A2011219632790
US Application 2005/0019158 (Claus) teaches the benefit of sheet metal turbine housings, with double walled design, in order, from the vehicle perspective, to save weight but more importantly, to reduce the thermal inertia of the system by preventing excessive cooling of the exhaust gases of the engine in the case of a turbocharger operating off peak cycle. These sheet metal turbine housings are used generally to enable the catalyst to reach operating temperature quickly, not to assist in Diesel particulate filter (DPF) regeneration. Sheet metal turbine housings reduce the mass and hence the thermal inertia but the tooling is very expensive as a separate stamping or forming tool has to be manufactured for each element of the turbine housing. The assembly and welding of the individual elements to make a complete turbine housing is also labor intensive and costly.
EP Application 1,541,826 B1 (Bjornsson) teaches the manufacture of a welded, lightweight, jacketed, exhaust manifold. Further, it is taught to be advantageous that the “efficient mass”, i.e. the mass that must be heated prior to catalyst light-off, is significantly reduced, since a smaller mass to be heated allows for a faster catalyst light-off. This patent does include the option of a wastegate but only in the sense that the pipe to the wastegate opening is jacketed and fabricated as part of the welded, jacketed manifold. The patent mentions that the wastegate o valve can be mounted in the wastegate at any occasion but preferably after all the welding is performed. The low thermal inertia feature of this design is favorable in reducing the catalyst light off period but the feature helping reduce the catalyst light off period still presents a reasonably high level of thermal inertia to the system for the process of DPF regeneration.
Thin walled turbine housing castings using the investment casting process are in use, and they do substantially reduce the mass and hence the thermal inertia, but at a significant increase in tooling and piece part cost. The cost of a welded sheet metal turbine housing in contrast is approximately 170% of the cost of a cast, ductile iron, housing and the weight savings is approximately 20%, so the thermal inertia will be approximately 80% that of a cast ductile iron turbine housing, but at a cost premium of 70%.
Over the past 20 years Diesel engine manufacturers have lowered NOx by 85% and particulate matter (PM) by 95%. For 2010 emissions, regulations mandate that emissions must be lowered a further 83%. The EPA intended for heavy-duty emissions, post 2007, to be “aftertreatment-forcing”. For light duty, tier 2 bin 5, emissions requirements are forcing aftertreatment today. This will require some novel approaches in order to satisfy all of these goals.
The typical exhaust composition of a modern gasoline engine comprises:                unburned hydrocarbons—HC        carbon dioxide and carbon monoxide—CO2 and CO        nitrogen and oxides of nitrogen grouped under the heading of N2 and NOx         unreacted oxygen—O2         
Since modern Diesel engines operate in a region very lean of stoichiometric, with an air/fuel ratio (A/F)>22, they produce carbon dioxide (CO2), and carbon monoxide (CO), gaseous oxides of nitrogen grouped under the heading of NOx, and hydrocarbons (HC).
The NOx segment of emissions is predominantly tackled by one of two methods: Exhaust Gas Recirculation (EGR) or Selective Catalytic Reduction (SCR). In either case the HC component is processed in a Diesel Particulate Filter (DPF).
For the reduction of THC, CO and TPM in Diesels, a Diesel Oxidation Catalyst (DOC) is commonly used. The DOC must be at a characteristic elevated temperature in order for it to work efficiently. The catalyst has to be at 210° C. before the catalyst functions above 25% efficiency, and it functions at 90% efficiency at 220° C. There have been test cycles for catalyst light off which demonstrate light off as low as 175° C. It is accepted that, to have a system which elevates the catalyst temperature to operating regimes, it must be capable of achieving between 175° C. and 210° C. in the first 60 to 120 seconds after cold start.
When the catalyst is at operating temperature, it converts some of the impurities in the exhaust gas, such as any unburned fuel, or combustion by-product, before the exhaust gas is emitted from the tailpipe into the atmosphere. The effectiveness of the catalyst, for the first few minutes of engine operation while still at ambient temperature, in a gasoline engine is almost non-existent. Between 60% and 80% of gaseous emissions are generated in these few minutes (some are as fast as 20 seconds) before the catalyst reaches its operating temperature of around 300° C. Gasoline engine catalysts operate at around 600° C. by the end of a trip. They generally will then cool back down to 300° C. within 30 minutes. Diesel catalysts are formulated for lower temperatures (200° C. to 300° C.). The majority of the generation of HC starts at about 20 seconds after engine start and continues at a growing high rate until 120 seconds after start.
The conversion of CO, by a catalyst, is temperature sensitive. FIG. 15 shows the conversion efficiency of a typical CO catalyst. The X-axis (143) depicts the temperature, in degrees centigrade, of the catalyst, at the catalyst. The Y-axis (144) depicts the conversion efficiency. It can be seen from the chart that the conversion efficiency (141) does not really begin until it kicks in (142) at a temperature of 220° C.
There are several existent solutions to this “time to cat-light-off” problem, one of which is using phase-change materials in the catalyst body to keep the catalyst substrate temperature close to the temperature required for the catalyst to function. Another is to close-couple the catalyst with the engine to minimize thermal inertia. Another development is to place a pre-catalyst prior to the turbine housing. There are many methods for dealing with this start-up non-thermally active catalyst problem. They are all complex, space invasive, and expensive.
U.S. Pat. No. 6,389,806 (Glugla) teaches that in order to shorten the time to reach operating temperatures an engine has variable displacement with retarded spark timing and air/fuel ratio biased to lean for the activated cylinder bank during and shortly afterward starting to further reduce the time required for catalyst light off.
U.S. Pat. No. 7,117,668 (Nashburn) teaches the use of a hydrocarbon reformer to supply the engine with fuel-lean reformate fuel mixture to ensure that all the fuel is burned while the exhaust converter is thermally non-functional.
U.S. Pat. No. 5,878,567 (Adamczyk) teaches a catalytic converter having a first highly loaded palladium or trimetal catalytic element containing palladium of relatively large particle size closely coupled to the engine exhaust manifold followed by one or more second catalytic elements having high oxygen storage capacity to provide protection against warmed up engine emissions break through is efficient in reducing cold start emissions through early catalyst light-off.
U.S. Pat. No. 5,410,872 (Adamczyk) uses an exhaust gas oxygen sensor to determine the amount of oxygen contained in the exhaust entering the catalyst, and an engine control computer connected with the air source and the oxygen sensor monitors the amount of oxygen contained in the exhaust and controls the amount of air supplied to the exhaust stream by the air source such that the available oxygen is slightly in excess of the stoichiometric requirement. In this manner, the light-off time of the catalyst is minimized.
There are devices which supply heat to the aftertreatment for the purposes of PM regeneration them or cleaning them. These devices are not intended to aid in faster catalyst light off, and are an added expense to the vehicle cost for PM regeneration. They deal with temperatures in the range of 700° C. to 800° C.
U.S. Pat. No. 3,908,371 (Tadashi) teaches a method of and a system for cleaning exhaust gases. The engine supplies exhaust gas proportioned to provide an excess-air ratio of about 1.0 to 1.15 so that substantially no carbon monoxide and hydrogen are present in the exhaust gases emitted from the engine. A reducing agent, such as hydrocarbons, is admixed to the exhaust gases entering the reducing catalyst for removing an excess of oxides and reducing the nitrogen oxides in the exhaust gases while secondary air is supplied to the exhaust gases entering the oxidizing catalyst for assisting in the oxidization of the carbon monoxide and hydrocarbons remaining in the exhaust gases passed through the oxidizing catalyst. This then cleans the catalyst at high temperature.
Diesel combustion also produces solids and liquids. These solids and liquids are usually grouped and referred to as particulate matter (PM). The PM component of diesel emissions comprises:                1. soluble organic fractions (SOF) from the lubricant        2. dry carbon (which is known as soot)        3. SOF from the fuel        4. SO3 and H2O        
According to the U.S. Environmental Protection Agency, 40 CFR Parts 9 and 86 “Test Procedures for Heavy-Duty Engines, and Light-Duty Vehicles and Trucks and Emission Standard Provisions for Gaseous Fueled Vehicles and Engines” PM is measured as part of the Federal Trade Procedure (FTP), in which an engine operates through a range of pre-determined cycles representing different driving cycles and gathers the PM for the entire cycle thus trapping the PM developed during engine transients. The PM portion of these emissions is often dealt with using Diesel particulate traps (DPF).
Most post-2007 US heavy duty Diesel engines come equipped with diesel particulate filters (DPFs). Catalyst-based DPFs, when used with ultra-low sulfur fuels, can achieve PM reductions in the region of 90%.
The DPF is a porous ceramic material housed in a metal housing located in the exhaust stream. The filter media in many of the commercially available DPFs is either a Cordierite or Silicon Carbide material. Typically the matrix has hundreds of channels; the opposite ends of adjacent channels are blocked thus forcing the exhaust stream through the tube sidewalls which captures the PM. The solid factions in the PM build up in the walls of the channels, causing blockage of the filter. The building up of the solid faction in the DPF is often referred to as “loading”. This solid faction has to be burned off to return the DPF to its properly functioning mode, and the process of burning the solid faction off is known as regeneration. The soot which burns off is converted to CO2 and CO, which then passes through the filter.
The soot from Diesel combustion also consists of non-combustible elements which deposit ash in the DPF. The cleaning of this ash from the DPF is called “cleaning” and must be done out of the vehicle. The frequency for cleaning is every 200,000 to 400,000 miles.
The engine exhaust temperature and duty cycle dictates whether the DPF action is passive or active. Passive regeneration requires that the continuous exhaust temperature is in the range of 325° C. to 420° C., which is sufficient to spontaneously combust the soot as it accumulates. When the DPF is operating in the passive mode the exothermic reaction emanating from the reaction between the PM, which is trapped in the channel walls and the oxidizing agents (O2 and NO2) will maintain the appropriate pressure differential in the DPF. In this case the DPF internal temperature is usually below 700° C.
Active regeneration requires the input of a person or system to initiate and accomplish regeneration. Active regeneration is required when the steady state exhaust temperature does not achieve sufficient temperature; the engine duty cycle does not allow enough high temperature to burn off the soot, or since the regeneration period is in the range of 20 to 30 minutes, the time for regeneration is too short. In this case the PM builds up, or loads in the channel walls, the pressure differential across the DPF goes up, which then usually triggers a command for the vehicle emissions system to go into the active regeneration mode.
In typical on-highway long haul trucks the DPF regeneration is mostly passive as the engine duty cycle is sufficient to develop enough exhaust gas temperature to continuously burn off the soot component of the PM.
In urban Diesel vehicle use, often the engine duty cycle is dictated by frequent starting and stopping, so both the temperature and time requirements for active regeneration are not met. In typical active regeneration systems the exhaust system is “dosed” with fuel for a period of time to elevate the temperature in the DPF for the regeneration period. This regeneration period is fraught with problems. For instance if the vehicle speed drops below a set speed, for example 20 mph, then the regeneration must stop. If the compression brake is activated, during regeneration, the additional fuel dosing must cease so the regeneration period is interrupted. Since the regeneration process develops temperatures in excess of 800° C., within the matrix, and consumes about a gallon of fuel, the process must be executed in a safe manner. These high temperatures can also be harmful to the substrate, which can warp and be damaged or even, in extreme cases, melt. If the substrate is coated, as is often the case, these extreme temperatures can damage the catalyst or the affixing of the catalysts to the substrate. If the regeneration temperature is not sufficiently high to support soot burn-off then the regeneration period is extended to durations in the 20 to 30 minute range.
The exhaust system of a modern Diesel engine usually has several after treatment devices in the system. In addition to the DPF, there can be Diesel Oxidation Catalysts (DOC); Ammonia Producers for SCR systems; SCR catalysts; and additional catalysts in the ammonia production system, all of which add thermal inertia to the system.
FIG. 2 depicts the layout of a typical single turbocharger installation in which fresh air is drawn in through an air cleaner (25), and then through an inlet duct (24) which fluidly couples the air cleaner (25) to the inlet of the compressor stage (2) of the turbocharger. The compressor stage (2) is driven by the turbine stage (1). Exhaust gas from the engine (100) passes from the combustion chamber of the engine to the exhaust manifold (7) to the turbine stage (1) and then passes to the after treatment devices through an exhaust pipe (11). The after treatment consists of a DPF (12) and a catalyst (13) but can also include the devices required to generate ammonia for an SCR system or for other after treatment devices. For an EGR system, the exhaust gas in the exhaust manifold is fed to the EGR cooler (15) and thence to the inlet manifold (22) where it mixes with the compressed air from the compressor stage of the turbocharger, after the compressed air is cooled in the intercooler (20). The mix of cooled, compressed air from the intercooler (20) and cooled EGR gas from the EGR cooler (15) is controlled by the inlet valve (21).
For DPF regeneration, either the combustion is dosed with excess fuel to generate a higher temperature exhaust flow, or in some cases an additional injector is provided just upstream of the DPF to “dose” the incoming exhaust flow to regenerate the DPF. In the case of upping the combustion temperature the fuel “dosing” elevates the typical engine-out temperature in the exhaust manifold to greater than 600° C. so that the temperature of the exhaust gas just upstream of the DPF exceeds 550° C. for at least 10 minutes for the regeneration period. The thermal inertia of all of the elements of the exhaust system, the turbocharger, the down pipe, and the exhaust pipe all work against the DPF seeing this elevated temperature efficiently and quickly, resulting in longer regeneration periods.
FIG. 6 shows an engine equipped with a regulated two stage (R2S) turbocharger system in which the exhaust gas flow can be modulated to flow through either or both, or a combination of both, turbine stages. The result of this layout, which is often used to generate high pressure ratios (in the compressor stage) required of EGR systems, is a more than double increase in thermal inertia since there are now two turbine stages and the ducting to facilitate that configuration. This means that the heat sink pulling thermal energy from the exhaust flow required for DPF regeneration is much greater and the regeneration period will be longer requiring more fuel for this regeneration process.
There are devices which supply heat to the aftertreatment for the purposes of PM regeneration. They deal with temperatures in the range of 700° C. to 800° C. U.S. Pat. No. 3,908,371 (Tadashi) teaches a method of and a system for cleaning exhaust gases. The engine supplies exhaust gas proportioned to provide an excess-air ratio of about 1.0 to 1.15 so that substantially no carbon monoxide and hydrogen are present in the exhaust gases emitted from the engine. A reducing agent, such as hydrocarbons, is admixed to the exhaust gases entering the reducing catalyst for removing an excess of oxides and reducing the nitrogen oxides in the exhaust gases while secondary air is supplied to the exhaust gases entering the oxidizing catalyst for assisting in the oxidization of the carbon monoxide and hydrocarbons remaining in the exhaust gases passed through the oxidizing catalyst. This then cleans the catalyst at high temperature.
It is imperative that the regeneration occur when commanded by the condition of the DPF so that the vehicle continues to meet emissions regulations. In most cases these conditions require that the process is performed in as short a time, at a temperature safe to the vehicle and DPF environment, as possible.
Thus there is a need for a simple, low cost solution, which lowers the thermal inertia of the exhaust system at either, and preferably both, the period of DPF regeneration and at the engine at start up.